Typically, a syringe comprises a hollow syringe cylinder which is open at a first end to accept a plunger, and which includes an axial port at a second end through which fluids may pass. The plunger may be of the forced discharge or of the displacement type, the displacement type having an intentional significant gap between the plunger and the syringe cylinder.
The forced discharge type of syringe relies mainly on a pressure differential between the syringe contents and the discharge port in order to force out the contents. Unfortunately, this type of syringe may display a non-linear relationship between plunger rate and discharge rate due in part to internal fluid dynamics, especially near the extremes of plunger travel. The displacement type of syringe generally displays a more linear relationship between plunger rate and discharge rate.
A common goal with many types of syringes is to eliminate gas within the hollow syringe cylinder. Displacement-type syringes may be generally more susceptible to gas build-up than other types mainly because of the substantial non-displaced volume remaining within a displacement-type syringe even at full plunger travel.
Syringes mounted in fluid-handling apparatus, such as apparatus for hematological analysis and/or particle counting, may be especially plagued by gas build-up due to the inability to conveniently reorient and reposition the syringe in order to expel a gas bubble. A significant problem with gas bubbles is that they act as pressure and vacuum reservoirs which especially reduce the displacement accuracy of the syringe. In addition, expelling the gas from a syringe mounted downwards may be nearly impossible with many prior art fluid-handling apparatus.
Accordingly, it is an object of the present invention to facilitate an improved displacement-type syringe suitable for mounting in fixed, fluid-handling apparatus and which overcomes the above-described drawbacks and disadvantages of the prior art.